unify exception handling into Utility.Exception

Removed old extensible-exceptions, only needed for very old ghc.

Made webdav use Utility.Exception, to work after some changes in DAV's
exception handling.

Removed Annex.Exception. Mostly this was trivial, but note that
tryAnnex is replaced with tryNonAsync and catchAnnex replaced with
catchNonAsync. In theory that could be a behavior change, since the former
caught all exceptions, and the latter don't catch async exceptions.

However, in practice, nothing in the Annex monad uses async exceptions.
Grepping for throwTo and killThread only find stuff in the assistant,
which does not seem related.

Command.Add.undo is changed to accept a SomeException, and things
that use it for rollback now catch non-async exceptions, rather than
only IOExceptions.

Assistant/XMPP/Client.hs

@@ -15,7 +15,6 @@ import Network.Protocol.XMPP
 import Network
 import Control.Concurrent
 import qualified Data.Text as T
-import Control.Exception (SomeException)
 
 {- Everything we need to know to connect to an XMPP server. -}
 data XMPPCreds = XMPPCreds
@@ -34,18 +33,18 @@ connectXMPP c a = case parseJID (xmppJID c) of
 
 {- Do a SRV lookup, but if it fails, fall back to the cached xmppHostname. -}
 connectXMPP' :: JID -> XMPPCreds -> (JID -> XMPP a) -> IO [(HostPort, Either SomeException ())]
-connectXMPP' jid c a = reverse <$> (handle =<< lookupSRV srvrecord)
+connectXMPP' jid c a = reverse <$> (handlesrv =<< lookupSRV srvrecord)
   where
 	srvrecord = mkSRVTcp "xmpp-client" $
 		T.unpack $ strDomain $ jidDomain jid
 	serverjid = JID Nothing (jidDomain jid) Nothing
 
-	handle [] = do
+	handlesrv [] = do
 		let h = xmppHostname c
 		let p = PortNumber $ fromIntegral $ xmppPort c
 		r <- run h p $ a jid
 		return [r]
-	handle srvs = go [] srvs
+	handlesrv srvs = go [] srvs
 
 	go l [] = return l
 	go l ((h,p):rest) = do